1. Field of the Invention
The present invention relates to a color image forming apparatus and a color correcting method for use in a color image forming apparatus, and more specifically, to a color image forming apparatus such as a digital color copying machine utilizing a masking method for toner color correction and a color correcting method for use therein.
2. Description of the Prior Art
In a conventional digital color copying machine, first, a light beam reflected from an original image is directed by a color scanner to a sensor having red, green and blue (hereinafter referred to as R, B G and B, respectively) spectral characteristics shown in FIG. 6, and formed into an R, G and B data.
The R, G and B data are, for example, before outputted to a printer portion of the copying machine, converted into data of yellow, magenta and cyan (hereinafter referred to as Y, M and C, respectively) which are complementary colors of R, G and B. The amounts (main densities) of Y, M and C toner provided onto a photoreceptor drum are determined according to the amount of reflection energy shown by the Y, M and C data. The main densities of the toner of Y, M and C which are complimentary colors of R, G and B are obtained by the following equations (1) to (3): EQU [Y]=1-[B] . . . . . . (1) EQU [M]=1-[G] . . . . . . (2) EQU [C]=1-[R] . . . . . . (3)
wherein:
[R] is a reflectance of R of an original image;
[G] is a reflectance of G of an original image;
[B] is a reflectance of B of an original image;
[Y] is a main density of Y toner;
[M] is a main density of M toner; and
[C] is a main density of C toner.
In practice, each toner has a spectral characteristic shown in FIG. 7. However, the above-mentioned equations (1) to (3) are premised on an assumption that each toner has an ideal spectral characteristic. For example, in a case of the M toner, the ideal spectral characteristic is M1 shown in FIG. 8. However, the M toner actually has a spectral characteristic M2 shown in FIG. 8. For this reason, a black image is formed if a processing of the color image data is made according to the equations (1) to (3).
To avoid this, a toner color correction is required.
In a digital color image forming apparatus, the toner color correction is made, for example, by a method where a color correction table of each of Y, M and C is used and a masking process is employed. In the method, each of the Y, M and C data are substituted in each masking equation, and a Y, M and C correction data which are automatically determined on determination of the Y, M and C data are provided as tables.
By this method, however, since processing is executed by use of data of 256 tones made by dividing each Y, M and C analog data into 256 tones, a large-capacity memory of 16 mega-byte for each color will be required if the tables are made of the Y, M and C data which are full data of 256 tones.
The use of the large-capacity memory results in a higher cost of the apparatus, more complicated circuit arrangement and reduction of processing speed. On the other hand, if the tone number is decreased so that a small-capacity memory is used, image reproducibility will deteriorate.